Chromium plating



United States Patent (9 3,303,114 CHROMIUM PLATING Edward A. Romanowski, Troy, Mich, assignor to The Udylite Corporation, Warren, Mich, a corporation of Michigan No Drawing. Filed Jan. 10, 1964, Ser. No. 336,868 22 Claims. (Q1. 204-51) This invention relates to the electrodeposition of chromium from aqueous acidic hexavalent chromium solutions, and especially to the use of the fluorine-containing compounds of tetravalent uranium in aqueous acidic hexavalent chromium solutions to make possible the deposition of improved chromium plate. More particularly it relates to the use of saturation concentrations of strontium sulfate with saturation concentrations of a tetravalent uranium fluoride in chromic acid solutions to make possible operational-1y simplified baths which give improved covering power in recessed areas (low current density areas). Not only is the chromium covering power improved, but no strong discoloration or iridescent films are formed where the chromium plate leaves off in the very low current density areas. Other advantages made possible by the use of saturation concentrations of fluorine-containing tetravalent uranium compounds in the chromic acid baths will be detailed hereinafter.

In order to electrodeposit chromium from acidic hexavalent chromium solutions, that is, chromic acid solutions, it was early established that it was necessary to have small amounts of certain anions such as sulfate or fluoride ions present and these ions were named catalyst ions. In the progress of the art of chromium plating from acidic hexavalent chromium baths, the use of mixed catalyst anions, such as combinations of sulfate ions with fluoride ions, sulfate with fluosilicate ions, sulfate with fluoborate ions, sulfate with fiuoaluminate ions, sulfate with fluotitanate ions, sulfate with fluozirconate ions and sulfate with boric acid were employed to make possible chromium plating baths of improved covering power, and of simpler control. In this connection, reference is made to the following United States Patents 1,844,751, 2,063,- 197, 2,042,611, 2,640,022, and 2,952,590. In particular, reference is made to Lukens United States 2,042,611, Passal United States 2,640,021, and Stareck United States 2,640,022 and 2,952,590, for the development of the selfregulation feature of the catalyst acid anion radical content of the chromium plating baths. Lukens used saturation concentrations of the slightly soluble strontium sulfate with or without added strontium chromate, to maintain the sulfate ion concentration in chromic acid baths containing about 200 grams per liter of CrO or less, Passal improved on this by also using with saturation concentrations of strontium sulfate, sparingly soluble potassium silicofluoride (fluosilicate), and Stareck made further improvements by using with the saturation concentrations of stontium sulfate and potassium silicofiuoride, mixed suppressing agents such as strontium chromate and potassium dichromate to better control the concentrations of sulfate and silicofluoride ions by common ion effects. Stareck also employed fluoaluminate, fluotitanate and fluozirconate ions instead of fluosilicate to use with controlled sulfate ion concentrations.

The standard or conventional chromium bath employing only the sulfate anion as catalyst and used for plating on nickel, iron, yellow brass or copper has been the 100 to 1 ratio of chromic acid anhydride (CrO to sulfate ion. Thus, in a 200 gram per liter CrO bath, 2 grams per liter of sulfate ion would be used, and in a 400 gram/ liter CrO bat-h, 4 grams/liter of sulfate ion would be used. If one uses instead a ratio of 2-00 to 1 of CrO to S0,, and plates on top of nickel, copper, yellow brass or steel,

3,303,114 Patented Feb. 7, 1967 only an iridescent non-metallic chromium chromate (rainbow) film instead of chromium plate is obtained. In particular, if chromium plating is attempted on top of a freshly plated bright nickel surface, from an aqueous solution containing, for example 340 grams/liter (45 oz./gal.) of chromic acid and saturated with strontium sulfate at, for example, about 50-52 C. R), which gives a ratio of about to 1 of chromic acid to sulfate and using dead entry into this chromic acid bath (that is, the direct plating current is turned on only after the nickel plated panel is immersed in the bath), no chromium plate is obtained on the nickel surface, only a non-metallic iridescent film of a basic chromic chromate results.

It has now been found that fluorine containing compounds of tetravalent uranium such as uranium tetrafluoride, uranium fluoborate, uranium fluosilicate, uranium fluoaluminate, uranium fluotitanate, uranium fluozirconate, which are partically insoluble in water and only slightly soluble in chromic acid solutions containing about 100 to 500 grams/liter of chromic acid, are unusual in cooperating with saturated strontium sulfate in the chromic acid baths to produce bright chromium plate of good covering power on nickel, brass, copper and steel. In this respect the uranium fluoborate and uranium fluozirconate are outstanding. The presence of the tetravalent uranium fluorides such as uranium fluoborate or uranium fluosilicate uranium fluozirconate in the bath saturated with the strontium sulfate makes possible the deposition of bright chromium plate over a very wide cathode current density range even with dead entry of the nickel plate into the chromium plating bath. The fluorine-containing compounds of tetravalent uranium can be used in concentrations of about 0.2 gram/liter to saturation concentrations. Saturation concentrations of the tetra valent uranium fluorides such as uranium fluozirconate, uranium fluoborate, etc., in the chromic acid baths do not cause any harmful effects in conjunction with saturation concentrations of stronium sulfate, such as greatly diminishing the covering and throwing power of the chromium deposit and producing dull white areas in the high current density chromium plate. In this very important respect, the saturation concentrations of the tetravalent uranium fluorides act quite differently than saturation concentrations of other inorganic fluorides which were tested, such as calcium fluoride, strontium fluoride, potassium or sodium fluosilicate (silicofluoride). These latter fluorides and silicoflu-orides when used in saturation concentrations in the chromic acid solutions very appreciably diminish the covering and throwing power of the chromium plate. It is for this reason that suppressing agents (for common ion effects) must be used in conjunction with saturation concentrations of these latter fluorides.

It was also found that adding boric acid up to saturation concentrations to the chronic acid baths containing uranium tetrafiuoride, or the uranium fluotitanate, or the uranium fluoborate or the uranium fluosilicate favorably affected the activation and catalyst properties of the tetravalent uranium fluorides especially that of the tetrafluoride and the fluotitanate. The boric acid extended the bright chromium plate coverage of the uranium tetrafluoride and the uranium fluotitanate very markedly. Adding fine silica powder or silicic acid or sodium silicate to the chromium plating baths containing the tetravalent uranium fluorides also favorably affected the activation and catalyst properties of the tetravalent uranium fluorides especially uranium tetrafluoride in the chromic acid baths containing saturation concentrations of strontium sulfate. In the case of boric acid, additions of about 1 gram/liter and especially about 5 grams/liter to saturation concentrations gave an appreciable cooperative effect especially with the uranium tetrafluoride and the uranium fluotitanate to extend the bright chromium plating coverage in recessed areas. The boric acid is somewhat superior to silicic acid in this respect. About 6 to 8 grams per liter of the tetravalent uranium fluorides appear to be more than sufficient to saturate the chromic acid baths. The best or optimum covering power chromium plating baths were obtained by the use in chromic acid baths containing about 300-350 grams/liter CrO of saturation concentrations of strontium sulfate and saturation concentrations (about 2 to about 6 grams/liter) of uranium fluozirconate or uranium fluoborate, or uranium fluosilicate, in the presence of perfluoro para ethyl cyclohexyl su-lfonic acid in concentrations of about 1 to 3 grams/liter, and boric acid at about to 30 grams /liter.

The saturation concentrations of the fluorine-containing tetravalent uranium compounds especially the fluoborate and the fluozirconate cooperate with saturation concentrations of strontium sulfate to give chromium plating baths of high covering power on nickel, steel, stainless steel, copper, brass and aluminum. As far as we have been able to ascertain this is the first time uranium fluorides have been used in acidic hexavalent chromium plating baths. The tetravalent uranium cation makes possible very simple regulation of the fluoride, silicofluoride (fluosilicate), fluoborate, fluozirconate, etc. ion concentrations in chromic acid solutions.

Instead of using strontium sulfate as the source of the sulfate ion, it is possible to use with the tetravalent uranium fluorides any source of sulfate ion that is less than about one hundredth the concentration of chromic acid and preferably in the ratio of about 120 to 1 to about 300 to 1 of chromic acid anhydride (CrO to sulfate ion.

The acidic hexavalent chromium plating baths may be made up from straight chromic acid anhydride or chromic acid, and from mixtures with dichromates, chromates, and po'lychromates. It is generally preferred to use straight chromic acid or chromic acid anhydride. The presence of cations such as Na, K, Li, Mg and Ca are not detrimental, but they are best kept in low concentration values. The important metallic cations are the metallic cations of this invention, the tetravalent uranium and the tetravalent uranium fluoride complexes to control the fluoride concentration which is normally difficult to control, unlike the sulfate concentration which is easy to control. Strontium ions may be present from strontium chromate or dichromate, but it is generally preferred that the strontium ions should be mainly from the saturation concentrations of strontium sulfate.

Thus, with the use of the saturation concentrations of the tetravalent uranium fluorides with saturation concentrations of strontium sulfate there is no need to use common ion effects to control the catalyst concentrations, that is, there is no need, for example, to use potassium dichromate in high concentrations to obtain a significant common ion effect with potassium silicofluoride when the latter is used as a co-catalyst with sulfate. The presence of high concentrations of potassium ions tends to cause the salting out of the important and valuable stable antimisting agent perfluoro n-octyl sulfonic acid, and this is a drawback in this respect, besides the fact that the suppressing salt is an added variable.

Below are listed several examples of baths of this invention:

Example I 200-400 grams/liter chromic acid anhydride (CF02) Saturation concentrations of SrSO (excess present) Saturation concentrations of uranium tetrafluoride (1-4 grams/liter) 5 grams/liter to saturation concentrations of boric acid 0 to 3 gram/liter of perfluoro para ethyl cyclohexyl sulfonic acid (1 to 3 grams/liter preferred) Temperature 115-130 F. (46-55 C.)

4 Example II 200-400 grams/liter CrO Saturation concentrations of SrSO (excess present) Saturation concentrations of uranium fluoborate or uranium fluozirconate (2-6 grams/liter, excess present) l-3 grams/liter of perfluoro para ethyl cyclohexyl sulfonic acid Temperature -l40 F.

Example Ill -400 grams /liter CrO Saturation concentrations of SrSO or 130 to 1 to 200 to 1 ratio of CrO /SO 0.4 to 2 grams/liter of uranium tetrafluoride or uranium tetrafluoborate or uranium tetrafluosilicate or uranium fluoaluminate or uranium fluoti-tanate or uranium fluozirconate or mixtures 0 to saturation concentrations of boric acid 0 to 20 grams/liter of calcium or cobalt zirconate 1-5 grams/ liter of perfluoro para ethyl (or methyl) cyclohexyl sulfonic acid (or the Na, K, CA, etc., salt) Temperature -140 F.

Example IV 200-400 grams/ liter CrO Saturation concentrations of uranium tetrafluoride (2-6 grams liter) 5-20 grams/ liter of calcium zirconate or cobalt zirconate Saturation concentrations of strontium sulfate (excess present) 1-3 grams/liter of perfluoro para ethyl cyclohexyl sulfonic acid Temperature 105-140 F.

The tetravalent uranium cation controls the fluoride ion whether in the form of a simple compound as uranium tetrafluoride or in a complex form as uranium tetrafluoborate, uranium tetrafluosilicate, uranium tetrafluozirconate, etc., or whether in the form of double salts, or oxysalts. The preferred salt is either the uranium fluoborate, or uranium fluozirconate, or uranium tetrafluoride added with boric acid or calcium zirconate or other zirconate salt. The concentration of the added zirconate radical from a zirconate salt can be from about 1 gram/liter to at least 30 grams/liter, though 5 to 10 grams/liter is the preferred concentration to 'be added to the chromic acid bath containing the uranium tetrafluoride. The zirconate radical improves the chromium coverage obtained with the UF compound present in the chromium bath. Almost any salt of the zirconate radical can be used through the barium salt is not preferred because it precipitates sulfate ion; it is the zirconate radical that is effective with the uranium tetrafluoride.

The tetravalent uranium fluorides can be best added as such to the chromium plating bath, for example, as uranium tetrafluoride, uranium tetrafluoborate, etc., or they can be formed in situ in the bath by reacting fluoboric acid, fluosilicic acid, hydrofluoric acid in the bath with uranium (tetravalent) hydroxide or hydrated oxide, UO (-H O) The oxide U 0 (especially in paste form) plus a reducing agent can also be used to form the tetravalent uranium fluorides and fluoride complexes. It is preferable to first form the uranium tetrafluoride, or the uranium fluozirconate or the uranium fiuoborate or uranium fluosilicate then make the addition to the bath, or UF can be added to the bath in saturation concentrations (l to 4 grams/liter) with a slight excess left over, and then boric acid, or silicic acid or sodium silicate, or sodium or magnesium or cobalt or nickel zirconate added in concentrations of preferably about 5 to 10 grams/liter.

Wit-h plating of thick chromium of 1 to 10 mils thickness or higher, the plate obtained is very bright with the saturation concentrations of a tetravalent uranium fluoride such as uranium fluozirconate, uranium fluoborate, etc.,

and saturation concentrations of strontium sulfate in chromic acid concentrations of 300-350 grams/liter, at bath temperatures of 11-5-12=5 F., also the plate is much less prone to stress-cracking. Thus, for crack-free bright chromium plate up to about 0.1 ml, it is less critical to use than many previous crack-free formulation. Also, it is suitable to use these chromium baths to obtain single or dual layers of micro-cracked chromium by using the more dilute concentrations of chromic acid such as 125 200 grams/liter.

What is claimed is:

1. A bath for the electrode position of chromium plate comprising an aqueous acidic hexavalent chromium plating solution containing a ratio of concentrations of CrO to sulfate ion greater than about 100 to 1, and less than about 300 to 1, and containing a fluorine containing tetravalent uranium compound in a concentration of about 0.2 grams/ liter to saturation concentrations.

2. A bath in accordance with claim 1 wherein said hexavalent chromium plating solution is chromic acid in a concentration of about 100 to about 500 grams/liter.

3. A bath in accordance with claim 1 wherein said uranium compound is uranium tetrafluoride.

4. A bath in accordance with claim 1 wherein said uranium compound is uranium tetrafluoborate.

5. A bath in accordance with claim 1 wherein said uranium compound is uranium tetrafluozirconate.

6. A bath in accordance with claim 1 wherein uranium compound is uranium tetrafluoaluminate.

7. A bath in accordance with claim 1 wherein uranium compound is uranium tctrafiuotitanate.

8. A bath in accordance with claim 1 wherein said uranium compound is uranium tetrafluosilicate (silicofluoride).

9. A bath in accordance with claim 1 wherein said sulfate ion is derived from saturation concentrations of strontium sulfate in said acidic hexavalent chromium plating solution.

10. A bath in accordance with claim 1 wherein said acidic hexavalent chromium plating contains boric acid in a concentration of about 1 gram/liter to saturation concentration.

11. A bath in accordance with claim 1 wherein said acidic hexavalent chromium plating solution contains the zirconate radical in a concentration of about 1 gram/ liter to about 30 grams/ liter.

12. A method of electrodepositing chromium which comprises passing current from an anode to a cathode to be coated in an aqueous acidic hexavalent chromium said said

plating solution containing a ratio of concentrations of CrO to sulfate ion greater than about to 1 and less than about 300 to 1, and containing a fluorine containing tetravalent uranium compound in a concentration of about 0.2 gram/ liter to saturation concentrations.

13. A method in accordance with claim 12 wherein said hexavalent chromium plating solution is chromic acid in a concentration of about 100 to about 500 grams/liter.

14. A method in accordance with claim 12 wherein said uranium compound is uranium tetrafluoride.

15. A method in accordance with claim 12 wherein said uranium compound is uranium tetrafluoborate.

16. A method in accordance with claim 12 wherein said uranium compound is uranium tetrafluozirconate.

17. A method in accordance with claim 12 wherein said uranium compound is uranium tetrafluoaluminate.

18. A method in accordance with claim 12 wherein said uranium compound is uranium tetrafluotitanate.

19. A method in accordance with claim 12 wherein said uranium compound is uranium tetrafiuosilicate (silicofluoride).

20. A method in accordance with claim 12 wherein said acidic hexavalent chromium plating solution contains boric acid in a concentration of about 1 gram/liter to saturation concentration.

21. A method in accordance with claim 12 wherein said acidic hexavalent chromium plating solution contains the zirconate radical in a concentration of about 1 gram/liter to about 30 grams/ liter.

22. A method in accordance with claim 12 wherein said sulfate ion is derived from saturation concentrations of strontium sulfate in said acidic hexavalent chromium plating solution.

UNITED STATES PATENTS References Cited by the Examiner 1,844,751 2/1932 Fink ct a1. 20451 1,928,284 9/1933 Fink et al. 204-51 2,042,611 6/ 1936 Lukens 204-51 2,063,197 12/ 1936 Schneidewind 204-51 2,640,022 5 3 Starcck 204-51 2,787,588 4/1957 Stareck et a1. 204-51 2,952,590 9/1960 Stareck et al. 204-51 2,962,428 11/ 1960 Passal 2045 1 JOHN H. MACK, Primary Examiner.

G. KAPLAN, Assistant Examiner. 

1. A BATH FOR THE ELECTRODE POSITION OF CHROMIUM PLATE COMPRISING AN AQUEOUS ACIDIC HEXAVALENT CHROMIUM PLATING SOLUTION CONTAINING A RATIO OF CONCENTRATIONS OF CRO3 TO SULFATE ION GREATER THAN ABOUT 100 TO 1, AND LESS THAN ABOUT 300 TO 1, AND CONTAINING A FLUORINE CONTAINING TETRAVALENT URANIUM COMPOUND IN A CONCENTRATION OF ABOUT 0.2 GRAMS/LITER TO SATURATION CONCENTRATIONS. 